Style of the initial bar line of a system.

In some engraving styles (mainly jazz), a double system initial bar line is used at structural articulations of the piece.

The PR supports this style, by adding a `_systemInitialBarLineStyle` variable to the `Measure` class which controls the style of the system bar line when the measure is the first of the system and by default it is set to NORMAL. This variable is not accessed directly; rather it is controlled by manipulating the system bar line. A system bar line can be edited:

- by dropping on it a bar line style from the palette (structural styles, like any repeat or END are ignored);
- by selecting it and double clicking a bar line style from the palette (same note as above).

It can be reset:

- by undo;
- by dropping on it (or selecting and double clicking) a NORMAL bar line style from the palette;
- by selecting and deleting it.

As the system bar line style is stored in the measure, if the first measure of a system changes (because of some other editing), the system bar line style will follow accordingly.
fix #32651: Naturals should not be shown at system break
album: don't add line break if page break
fix #40171: Crash in ~ScoreView() fixed
fix #6535: grace note ties, articulations, and undo fixes
add vtest
fix #40301: ties can overlap staff lines
fix #40291: voice 3/3 rests in rhythmic slash notation
Merge pull request #1488 from MarcSabatella/40301-tie-staff-line

fix #40301: ties can overlap staff lines
fix #40321: inappropriate part names in Classic Orchestra template

changed all partnames to match the long instrument names
fix #40226: Add string quartet template
enable menu for resource-manager unconditionally
fix #40296: lyrics consisting of symbols offcenter
add mtest
update the style definitions to match the MusicXML word-font and lyric-font
set size only for specific types of text
testfiles for font handling
Merge pull request #1494 from lvinken/musicxml-font-handling

Musicxml font handling
Merge pull request #1491 from Jojo-Schmitz/resource-manager

enable menu for resource-manager unconditionally
Merge pull request #1489 from MarcSabatella/40291-rhythmic-notation-rests

fix #40291: voice 3 rests in rhythmic slash notation